In modern handheld devices for cellular communication systems (e.g. 3GPP) there is a desire to support multiple frequency bands (e.g 3GPP LTE bands 1, 2, 3, 5, 7, 8, and 13). Further in this regard, one particular area of interest in radio communication has been how to deal with transmitter and receiver duplexing, with transmitter and receiver operating at a fixed or variable frequency separation. For such frequency division duplex, an issue known as duplex self-interference can present a problem in the design of such systems. The problem arises from the high power of the transmitter challenging the linearity of the receiver that can be set up to have a high gain to deal with low power reception levels.
For small handheld devices, duplex operation has typically been achieved using fixed frequency filters or duplex filters (e.g., dielectric coaxial resonator filters, SAW, BAW, FBAR) that are switched among operating frequencies (e.g., for multiband operation) using semiconductor switches due to technology and size constraints. These fixed frequency filters and antennas exhibit particular limitations in that, for each band of operation, a new set of hardware must be introduced (e.g., adding antenna resonator coupling element, filters, and switches when adding band support). As a result, tunable systems would be beneficial in reducing the amount of hardware required to operate at a range of frequencies, but it is difficult to make a tunable system also cost effective and small, while at the same time meeting system requirements (e.g., 3GPP standards). In addition, the location of filters in present days phones also presents a limitation in that filters have had a design constraint on component height of less than 1 mm so it could be placed together with RF transceiver IC, digital processing IC, and multimedia processing IC.
Accordingly, it would be desirable for there to be a solution to bring down the size of a tunable solution, to make it cost efficient, and at the same time, with proper design, to solve the issue of removing unwanted interference, such as from a transmitter in the wireless communication terminal. In addition, it would further be desirable for a combined solution that makes it acceptable to place the filter in a location on the phone board that allows higher building height, and therefore allows higher diameter inductors to increase inductor Q and thereby make tunable frequency filter characteristics acceptable to system requirements.